The qualitative and quantitative aspects of IgE production in vitro by human peripheral blood lymphocytes are being investigated. Human B lymphocytes, cultured in vitro, can be shown to be under the regulatory influence of helper and suppressor T lymphocytes. Abnormalities in the activity of these regulatory cells in conditions of altered IgE production are being sought. In addition the number of IgE forming B cells in normals is being assessed and compared to that in states of altered IgE production. The markers of these IgE producing cells is being evaluated by surface Ig isotype analysis and the effect of various anti-isotypes on subsequent IgE production. Whether the B cells that produce IgE in vitro in atopic and immune patients with markedly elevated IgE levels are a different subpopulation than the B cells found in normals is being investigated. Their being functionally distinct subsets is suggested by their IgE synthesis in the absence of pokeweek mitogen and T cells.